Current permission checking mechanism doesn't return its error code
well. The internal error (code = 13) is returned to client and the
retry mechanism doesn't work well. This commit fixes the problem.
This commit implements Authenticate() API of the auth package. It does
authentication based on its authUsers bucket and generate a token for
succeeding RPCs.
This commit adds a functionality of user deletion. It can be invoked
with the new user delete command.
Example usage:
$ ETCDCTL_API=3 etcdctl user delete usr1
